How to turn a nikon d3100's settings into default?

Someone played with my camera's settings so I'm really not able to use the camera like I used to so I would really need some help

Check the manual for a reset option.

If you don't have a paper manual, you can download a free pdf from Nikon.

Time to read the manual.

Look on page 95 of your user manual

In the future, until you understand what various functions do, only change one at a time